With components of rotary machinery, such as a gas turbine engine, a consistent roundness (defined as a constant radius about a point or an axis) is difficult to obtain. A relatively inflexible cylindrical part, like a rotor, can be made very close to round but the part may be subject to material flaws and malformations, handling and assembly, and operating parameters that affect the constancy of its defining radii fairly constantly throughout the part.
Relatively flexible parts, like a blade or a casing complicate the issue because of their greater susceptibility to damage and motion during manufacture, assembly and use. For example, as blades rotate about a rotor, their rotating blade tips define a desired substantially cylindrical envelope in which the blades rotate. However, the blade lengths may not be equal, the blade radii (and their supports) lengthen and shorten as engine operating temperatures vary and the blades may flex under load.
Similarly, a thin, relatively flexible, stationary casing is disposed around the substantially cylindrical envelope. For efficiency, it is desired that this casing be closely aligned with the envelope to prevent air or other gasses from escaping around the blade tips. However, the casing may not react to temperature changes in the engine in the same manner as the blades and the rotors and is subject to other loads in the engine. Control systems may be used in the engine to keep the casing closely aligned with the cylindrical envelope. Such systems, however, may not be perfect and some blade tip-to-casing interference may occur.
During operation, especially when the engine is newer, the engine may define for itself its own definition of roundness and minimize out of roundness as parts interact and contact each other. Abradable coatings are used to protect the parts as interaction occurs. Some blades have coatings or tip treatments that affect the wear of the blades during operation.